How many species are there on Earth? Progress and problems

How many species exist on Earth? Projections range from millions to trillions. A 2011 paper in PLOS Biology provided a comprehensive estimate of 9 million.

for 18 groups in which total species numbers (described and undescribed) were considered "relatively well known."They found a strong relationship between these estimates.
The study by Mora and colleagues [4] has become very widely cited and used, and deservedly so.Yet, it would be problematic to treat their estimates as the final word on global biodiversity and especially to reject new estimates simply because they are larger.Mora and colleagues generally did not incorporate the vast species numbers revealed through molecular analyses (e.g., metabarcoding).Below, I review published diversity projections for several key groups that are dramatically larger than those from Mora and colleagues, with species revealed by molecular data largely driving these bigger estimates (Table 1).
Mora and colleagues [4] estimated approximately 10,000 bacterial species (roughly the number of described species).They acknowledged that these projections were likely underestimates.Yet, prokaryotes may be a major driver of Earth's overall species richness.Recent studies have estimated a staggering range of species numbers for bacteria, from low millions [6], to hundreds of millions [10], to low trillions [3].All were based on extrapolations from molecular studies.Clearly, controversies about global biodiversity cannot be resolved without better resolving bacterial richness.
For protists, Mora and colleagues [4] estimated 63,900 species.Yet, Adl and colleagues [7] estimated 1.2 to 10 million species, almost all in Apicomplexa (including the malaria-causing Plasmodium).This projection was based on the "number of unknown DNA sequences found in environmental samples."However, no specific methodology was given for these extrapolations, which makes them difficult to rigorously evaluate.
For fungi, Mora and colleagues estimated 611,000 species.Nonetheless, estimates of global fungal diversity from environmental sequencing methods have ranged into the millions for years now [11].A recent study [8] combined estimates from 335 million comparable nucleotide sequences from >200 studies.They found 1.1 million putative species, after excluding a remarkable 9.5 million potential species each represented by a single sequence.These putative species were mostly Ascomycota (57%; yeasts and relatives) and Basidiomycota (37%; mushrooms and relatives).They extrapolated from this sampling to conservatively project 6.3 million fungal species.
Many studies have estimated the total number of insect species.Their projections have been comfortingly similar for decades, often around 6 million species [12].This number is similar to the 5.6 million terrestrial animal species estimated by Mora and colleagues [4].However, these estimates have not explicitly incorporated morphologically cryptic species revealed by molecular analyses.Recent analyses suggest that each insect species initially delimited by morphology might conceal (on average) 3.1 cryptic species [9].Combined with projections of approximately 6 million morphology-based insect species, this yields estimates of approximately 20 million insect species [9].Nevertheless, these extrapolations could be overestimates for two main reasons.First, large-scale barcoding studies have not found as many cryptic species as have studies of individual species [12].However, it is unclear if this discrepancy is caused by limited geographic sampling within species in large-scale barcoding studies or instead by biased selection of species for studies of individual species (but see [9]).The second problem in these extrapolations is the assumption that undescribed insect species harbor as many cryptic species as described species.Instead, undescribed species might be more narrowly distributed than typical described species and thus might contain fewer cryptic species.
Both problems require further study.Another complication is that the diversity of some taxonomic groups might depend on other organisms.A review suggested that each insect species might host (on average) a unique species of mite, nematode, apicomplexan protist, and microsporidian fungus, and several bacteria [10].These inferences were based on case studies that focused on species in these groups hosted by closely related insects.When combined with larger projections of insect diversity, these insect-associated species could push global biodiversity past 100 million species, with tens of millions from these five groups.Some readers might reasonably be squeamish about projecting such enormous numbers based on relatively few case studies.What is therefore needed are additional studies of closely related insect species to document the number and specificity of their host-associated species from these five groups (and possibly others).
In summary, Mora and colleagues [4] made a transformative contribution to the study of Earth's biodiversity.They combined existing biodiversity databases with rigorous statistical methods to produce one of the first comprehensive estimates of species numbers spanning all major groups.Yet, new molecular data are dramatically increasing richness estimates for many of these groups (Table 1).Our estimates of global biodiversity should continue to evolve as they incorporate these new types of data.
Unfortunately, estimates of global biodiversity may soon be changing fundamentally in another way.Global biodiversity is now facing numerous threats.The most important ones may be habitat destruction and overexploitation [13], and climate change can threaten even protected species in well-preserved habitats [14].Soon, we may not be estimating how many species there are on Earth.We will be estimating how many there were.